In recent years there has been growing interest in organic electronic (OE) devices, for example field effect transistors for use in display devices and logic capable circuits, or organic photovoltaic (OPV) devices. A conventional organic field effect transistor (OFET) typically includes source, drain and gate electrodes, a semiconducting layer made of an organic semiconductor (OSC) material, and an insulator layer (also referred to as “dielectric” or “gate dielectric”), made of a dielectric material and positioned between the OSC layer and the gate electrode.
A broad range of different substrates can be used for OE devices like OFETs and OPVs. The most common are polymers like polyethylene terephthalate (PET), polyethylene naphthalate (PEN), other polyesters, polyimide, polyacrylate, polycarbonate, polyvinylalcohol, polycycloolefin or polyethersulphone. Thin metal films, paper based substrates, glass and others are also available.
However, the substrates that have hitherto been available often contain defects and contamination from the production process. Therefore, for the purpose of integrity of the thin-film OE devices made on top of them, most of these substrates require an additional planarization or barrier layer in order to provide a smooth and defect-free surface.
Further reasons or functions requiring the application of an intermediate layer between substrate and OSC material include: 1) improving the hardness/scratch resistance of the substrate, 2) providing electrical isolation of the substrate and the OSC layer, 3) providing a barrier to prevent diffusion of metal ions, small molecules, and oligomers from the carrier substrate to OSC, 4) modifying wetting properties of the substrate, and 5) acting as adhesion promoter.
Various plastic film substrates are commercially available, like for example PET films of the Melinex® series or PEN films of the Teonex® series, both from DuPont Teijin Films™.
Typical commercially available planarization, hard-coating, or barrier materials include:
1) Silicon dioxide (SiO2) or silicon nitride (SiNX) electrical insulators, which are used mainly on top of conducting metal substrates.
2) Organic polymers, such as, acrylic-, melamine- or urethane-based polymers.
3) Organic-inorganic hybrid composites, which are based mainly on the use of metal alkoxide and organosiloxane via sol-gel processing, as disclosed for example in U.S. Pat. No. 5,976,703 or in W. Tanglumlert et al. ‘Hard-coating materials for poly(methyl methacrylate) from glycidoxypropyl-trimethoxysilane-modified silatrane via sol-gel process’, Surface & Coatings Technology 200 (2006) p. 2784.
Nevertheless, to date there has been no planarization material which fulfills all requirements for all the commercially available OE/OPV materials. Two of the major weaknesses of the currently available materials are: 1) a low surface energy, which causes de-wetting of OSC materials during coating, therefore requiring additional pre-treatment, and 2) a high permeation of the available polymers and composites to water. Therefore, the above-mentioned materials are not suitable for many OE/OPV applications unless an additional barrier or surface modification layer is applied.
Moreover, the inventors have found that the planarization materials used in commercially available PET or PEN substrates have turned out not to be fully compatible with recently developed high performance OSC materials, like those of the Lisicon® Series (commercially available from Merck KGaA or Merck Chemicals Ltd.). Further, poor electrical stability of devices using the Lisicon® Series OSC directly on top of planarised Melinex® and Teonex® has been observed. Therefore, an additional barrier/surface modification layer on top of the existing planarization layer, or a replacement for the planarization layer would be advantageous.
In general, a planarization material should exhibit one or more of the following characteristics:
1). acting as an electrical insulator,
2). providing a smooth surface (preferably arithmetic average roughness of absolute values (Ra)<5 and maximum high of the profile (Rt)<50),
3). providing for the electrical performance and stability of OTFTs compared to the best working example on any other substrate,
4). enabling good adhesion between the substrate and electrode metals (preferably 5N/cm or higher),
5). possessing good wetting properties for OSC formulations (preferably a surface energy of the planarization layer ≧50 mN/m),
6). inherent resistance to process chemicals,
7). optical transparency in the visible spectrum,
8). deposition using well established industrial processes.
Therefore, there is still a need for improved planarization layers which can be used in OE devices, especially OFETs and OPV cells, which fulfill the above-mentioned requirements.
One aim of the present invention is to provide planarization layers meeting these requirements. Another aim is to provide improved OE/OPV devices comprising such planarization layers. Further aims are immediately evident to the person skilled in the art from the following description.
The inventors of the present invention have found these aims can be achieved by providing planarization layers and OE devices in accordance with the present invention and as claimed hereinafter.